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nebula-mesh: Host advanced overrides allow YAML injection into agent config.yml

High severity GitHub Reviewed Published May 20, 2026 in forgekeep/nebula-mesh • Updated Jun 8, 2026

Package

gomod github.com/juev/nebula-mesh (Go)

Affected versions

< 0.3.2

Patched versions

0.3.2

Description

internal/configgen/generator.go:86,108,119 interpolates the operator-supplied ListenHost and TunDevice fields raw into a text/template that produces the agent's config.yml. internal/web/advanced.go:20-35 accepts both with only strings.TrimSpace — no character or shape validation.

Exploit

An operator (or attacker with any operator key, given the cross-tenant CRUD advisory) sets adv_tun_device to:

nebula0
lighthouse:
  am_lighthouse: true
  hosts: ["10.0.0.1"]
#

The agent fetches the rendered config on its next signed poll. On config reload, it loads the injected YAML keys: the host self-promotes to lighthouse, attracts mesh traffic, or sets am_relay: true to be selected as a relay. The ListenHost field has the same shape.

Affected

All released versions prior to v0.3.2.

Threat model

  • Today: operator can compromise their own host's config (trivially allowed if they own the host, but they can also set lighthouse/relay flags that the operator-create form does NOT expose — privilege uplift within their own tenant).
  • Combined with the critical /api/v1 authz advisory: any operator key can mutate ANOTHER tenant's host overrides and inject YAML there.
  • Post-fix of the authz advisory: still relevant — the agent unconditionally trusts whatever config the server hands it, so any future operator-impersonation bug re-amplifies this.

Suggested fix

Two options, either acceptable:

  1. Input validation in parseAdvancedFromForm (internal/web/advanced.go):

    • ListenHost: regex ^[A-Za-z0-9.:\[\]_-]+$ (IPv4/IPv6/hostname)
    • TunDevice: regex ^[A-Za-z0-9_-]{1,15}$ (Linux IFNAMSIZ caps at 15)
      Reject invalid input with a form-level error; do not write to the host row.

  2. Safer marshalling: switch configgen/generator.go to marshal a typed Go struct via gopkg.in/yaml.v3 (which escapes correctly) instead of text/template string-concat. Larger change, but eliminates this entire injection class.

Option 2 is preferable long-term. Option 1 is the quick fix.

The unsafe_routes advanced field is already netip.Parse{Prefix,Addr}-validated at enroll.go:226-233 — apply the same validation discipline to the other advanced fields.

References

@juev juev published to forgekeep/nebula-mesh May 20, 2026
Published to the GitHub Advisory Database Jun 8, 2026
Reviewed Jun 8, 2026
Last updated Jun 8, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

EPSS score

Weaknesses

Improper Control of Generation of Code ('Code Injection')

The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment. Learn more on MITRE.

CVE ID

CVE-2026-47722

GHSA ID

GHSA-7hp6-g3pq-3pc3

Source code

Credits

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